1. Field of the Invention
The present invention relates to interferometer-based alignment and position measurement systems. More particularly, the invention relates to such systems used in lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
patterning means, for patterning the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a projection system for imaging the patterned beam onto a target portion of the substrate.
2. Description of the Related Art
The term xe2x80x9cpatterning meansxe2x80x9d should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d has also been used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask table for holding a mask. The concept of a mask is well known in lithography, and its includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The mask table ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaking on the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask table and mask; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
Also for the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The illumination system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and U.S. Ser. No. 09/180,011, filed Feb. 27, 1998 (WO 98/40791), incorporated herein by reference.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a stop-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
There is a continuing desire in the semiconductor industry to be able to manufacture integrated circuits (ICs) with ever higher component densities and hence smaller feature size. To image smaller features in a lithographic projection apparatus it is necessary to use projection radiation of shorter wavelength. A number of different type of projection radiation have been proposed, including Extreme Ultraviolet (EUV) in the 10-20 nm range, electron beams, ion beams and other charged particle fluxes. These types of radiation beam share the requirement that the beam path, including the mask, substrate and optical components, be kept in a high vacuum. This is to prevent absorption and/or scattering of the beam and a total pressure of less than about 10xe2x88x926 millibar is necessary. Optical elements for EUV radiation can be spoiled by the deposition of carbon layers on their surface which imposes the additional requirement that hydrocarbon partial pressures must be kept below 10xe2x88x928 or 10xe2x88x929 millibar.
Working in such a high vacuum imposes quite onerous conditions on the components that must be put into the vacuum and on the vacuum chamber seals, especially those around any part of the apparatus where a motion must be fed-through to components inside the chamber from the exterior. For components inside the chamber, materials that minimize or eliminate contaminant outgassing, either from the materials themselves or from gases adsorbed on their surfaces, must be used.
It also is well known that the substrate (wafer) that is being exposed must be positioned to extremely high accuracy relative to the mask (reticle). A wafer may undergo 20 or 30 exposures during the manufacture and it is essential that the various images are properly aligned, even if different lithography apparatus are used for different exposures. The overlay accuracy requirements only increase with reduced feature size and shorter wavelength radiation.
An object of the present invention is to provide an alignment and/or position measuring system capable of measuring the position of an object in vacuum with high accuracy, e.g. for use in a lithographic projection apparatus.
According to the present invention there is provided a lithographic projection apparatus comprising:
a an illumination system for supplying a projection beam of radiation;
patterning means, for patterning the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a projection system for imaging the patterned beam onto a target portion of the substrate; characterized by:
a vacuum chamber in which at least one of said patterning means and said substrate table is contained, said object table being movable; and
an alignment system constructed and arranged to align said patterning means and a substrate on said substrate table, said alignment system comprising a passive part contained in said vacuum chamber and an active part outside said vacuum chamber.
By positioning only the passive part of the alignment system inside the vacuum chamber, the present invention avoids difficulties in making the active part of the alignment system vacuum compatible and reduces heat and vibration generation in the vacuum system, which may disturb the exposure and cause positioning uncertainties.
In embodiments of the present invention, the active and passive parts of the system are coupled together by optical fibers. The alignment system may be an interferometer system for detecting the position of a measurement grating (wafer mark) relative to a reference grating. Such a system may image at least two orders of radiation diffracted by the measurement grating onto the reference grating and may comprise beam splitting means to separate radiation diffracted by the reference grating and deriving from one order diffracted by the measurement grating from diffracted radiation deriving from other orders diffracted by the measurement grating.
According to a further aspect of the invention there is provided a device manufacturing method comprising the steps of:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using an illumination system;
using patterning means to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material; and
providing a vacuum chamber which comprises a movable substrate table for holding the substrate; characterized by the step of:
prior to or during said step of irradiating and imaging, aligning said patterning means and substrate on said substrate table using an alignment system comprising a passive part provided in said vacuum chamber and an active part provided outside said vacuum chamber.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet (UV) radiation (e.g. at a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), extreme ultraviolet (EUV) radiation, X-rays, electrons and ions.